Porous monolithic structures have evolved into versatile carrier materials in a wide range of flow-through applications in chemical analysis, biosciences, catalysis, etc. (Svec et al., eds. “Monolithic materials”, ISBN-13:978-0-444-50879-9, Elsevier 2003) Such organic monoliths are almost invariably made by direct mould polymerization of precursor monomers (commonly vinylic, but more recently also epoxy-based condensation systems) in the presence of porogens, which establish flow-through and diffusive pores in the material. Direct polymerization in situ is simple and has many advantages, but also drawbacks such as difficulties in establishing a spatially homogenous pore size distribution due to thermal gradients caused by exothermic polymerization reactions, since the pore formation mechanism is highly sensitive to the polymerization temperature. The decrease in specific volume when monomers are converted to polymers also causes shrinkage and related strain, which can explain structural breakdown or detachment from the mould walls, as often seen in monoliths directly polymerized from vinylic monomers. The primary object when preparing a monolith for separation purposes is to establish a pore system with evenly spaced equi-sized pores. In conventional direct monolith polymerizations, this pore formation is effected by the non-polymerizable porogens, which are selected to be a good solvent for the monomer but an intermediate to bad solvent for the polymer formed.
Accordingly, there is a need for an improved method for producing porous monolithic structures that does not have the drawbacks mentioned above.
Numerous polymers can be brought into solutions by solvents, and the parameters that determine the swelling and eventual dissolution is the solvent quality and Θ-temperature in the polymer/solvent system in question. Dissolution/precipitation is a technique often used for preparing membranes, where one face of the cast polymer solution is open to the further treatment (evaporation, solvent treatment, etc.). The means most often used for controlling the polymer precipitation rate (establishing deviation from Θ-conditions) is selective vaporization or treatment with a non-solvent, used as a liquid bath or deposited onto the membrane from the gas phase.
Polyamide is among the polymers that have found widest uses as membranes prepared by dissolution/precipitation and the applications span the environmental, biotechnological, and medical sciences. Examples of such applications are in desalination by reverse osmosis (Shibata et al., Journal of Applied Polymer Sciences 2000, 75, 1546-1553), waste water treatment (Sheriff et al., J. Agric. Food Chem. 2002, 50, 2802-2811), affinity chromatography for protein purification (Castilho et al., Journal of Membrane Science 2000, 172, 269-277; Castilho et al., Journal of Membrane Science 2002, 207, 253-264; Beeskow et al., Journal of Colloid and Interface Sciences 1997, 196, 278-291), microfiltration (Persson et al., Journal of Membrane Science 2003, 223, 11-21; Yang et al., Chem. Eng. Technol. 2006, 29, 631-636; Gholap et al., Journal of Membrane Science 2001, 183, 89-99), and hemodialysis (Mochizuki et al., Journal of Applied Polymer Sciences 1997, 65, 1713-1721). The popularity of polyamide membranes is a result of material durability, wide range of hydrophilic-hydrophobic (amphiphilic) properties and numerous possibilities for functionalization. Introduction of surface functional groups can be made by chemical conversion or by thermal and photo-induced (Teke et al., Process Biochemistry 2006; Wu et al., Journal of Membrane Science 2006, 283, 13-20; Eldin et al., Advances in Polymer Technology 1999, 18, 109-123) grafting, and modifications that target the main chain amide groups (Jia et al., Polymer 2006, 47, 4916-4924; Herrera-Alonso et al., Langmuir 2006, 22, 1646-1651) are especially attractive due to the possibilities of reaching a higher ligand density than those based on terminal amino and/or carboxylic groups. Prevention of non-specific interactions of the materials with biomolecules, i.e. establishing biocompatibility, can be improved by immobilization of layers of hydrophilic polymers such as dextran or chitosan (Shi et al., Journal of Chromatography B 2005, 819, 301-306; Xia et al., Journal of Membrane Science 2003, 226, 9-20), or by incorporating polyethylene into the polyamide matrix (Mochizuki et al., Journal of Applied Polymer Sciences 1997, 65, 1713-1721).
Polyamide membranes are made by a dissolution/reprecipitation (wet phase inversion) process and there are several ways to bring about phase separation of a polyamide solution when such membranes are manufactured, normally by a casting or extrusion procedure. One such route is to dissolve the polymer in a single good solvent or a mixture of an intermediate and a good solvent, and then exposing the cast precursor polymer solution to a non-solvent, leading to coagulation. This solvent-induce precipitation can be effectuated either by immersion in a liquid bath or by exposure to saturated vapor of a non-solvent. Other ways of inducing precipitation is through selective evaporation, by selecting a solvent mixture such that the good solvent is the more volatile member of a solvent pair, or by lowering the temperature below the upper critical solution temperature (UCST). Among these techniques, precipitation by treatment with a non-solvent seems to be the most popular. The solvent/non-solvent system employed for solvent-induced precipitation of polyamides 6 and 66 is usually formic acid/water (Shih et al., Journal of Applied Polymer Sciences 2005, 96, 944-960), although in some cases more exotic solvent mixtures such as 2,2,2-trifluoroethanol/compressed CO2 (Kho et al., Polymer 2001, 42, 6119-6127) are used. Films of polymer dope solutions cast on surfaces that do not form adhesive bonds to the polymer can be directly immersed into a non-solvent bath and the polymer precipitates by the exchange of solvents by non-solvents to form porous membranes. Studies of the mechanism of membrane formation has had great impact on the compositions of polymer solutions and non-solvent baths on the membrane morphology (Shih et al., Journal of Applied Polymer Sciences 2005, 96, 944-960; Wienk et al., Journal of Membrane Science 1996, 113, 361-371). The resulting membranes have different morphologies depending on whether a liquid-liquid or a solid-liquid phase separation occurs first.